5-Lipoxygenase is the first dedicated enzyme in the pathway leading to the biosynthesis of leukotrienes (Samuelsson, B., Science, 120: 568 (1983); Hammarstrom, S., Annual Review of Biochemistry, 52: 355 (1983)). This important enzyme has a rather restricted distribution, being found predominantly in leukocytes and mast cells of most mammals. Normally 5-lipoxygenase is present in the cell in an inactive form; however, when leukocytes respond to external stimuli, intracellular 5-lipoxygenase can be rapidly activated. This enzyme catalyzes the addition of molecular oxygen to fatty acids with cis,cis-1,4-pentadiene structures, converting them to 1-hydroperoxy-trans,cis-2,4-pentadienes. Arachidonic acid, the 5-lipoxygenase substrate which leads to leukotriene products, is found in very low concentrations in mammalian cells and must first be hydrolyzed from membrane phospholipids through the actions of phospholipases in response to extracellular stimuli. The initial product of 5-lipoxygenase action on arachidonate is 5-HPETE which can be reduced to 5-HETE or converted to leukotriene A4 (LTA4). This reactive leukotriene intermediate is enzymatically hydrated to LTB4 or conjugated to the tripeptide glutathione to produce LTC4. LTA4 can also be hydrolyzed nonenzymatically to form two isomers of LTB4. Successive proteolytic cleavage steps convert LTC4 to LTD4 and LTE4.
Other products resulting from further oxygenation steps have also been described (Serhan, C. N., Hamberg, M., and Samuelsson, B., Proceedings of the National Academy of Sciences, U.S.A., 81: 5335 (1985); Hansson, G., Lindgren, J. A., Dahlen, S. E., Hedqvist, P., and Samuelsson, B. FEBS Letters, 130: 107 (1984)).
Products of the 5-lipoxygenase cascade are extremely potent substances which produce a wide variety of biological effects, often in the nanomolar to picomolar concentration range. (Sirois, P., Advances in Lipid Research, R. Paoletti, D. Kritchevesky, editors, Academic Press, 21: 79 (1985).
The remarkable potencies and diversity of actions of products of the 5-lipoxygenase pathway have led to the suggestion that they play important roles in a variety of diseases. Alterations in leukotriene metabolism have been demonstrated in a number of disease states. Examples of some of these are briefly discussed as follows:
1. Asthma. Slow reacting substance of anaphylaxis (SRS-A) has long been recognized as a potentially important mediator of asthma and other allergic diseases (Orange, R. P. and Austen, K. F., Advances in Immunology, 10: 105, 1969). Upon specific antigen challenge, tissues from allergic animals and humans generate and release SRS-A (Kellaway, C. H. and Trethewie, E. R., Quarterly Journal of Experimental Physiology, 30: 121, 1940; Orange, R. P., Stechschulte, D. J., and Austen, K. F., Journal of Immunology, 105: 1087, 1979; 9. Lewis, R. A., Wasserman, S. I., Goetzl, E. J., and Austen, K. F., Journal of Experimental Medicine, 140: 1133, 1974). It produces a slow and sustained contraction of airway smooth muscle preparations from a variety of species in vitro, including man (Drazen, J. M., Lewis, R. A., Wasserman, S. I., Orange, R. P., and Austen, K. F., Journal of Clinical Investigation, 63: 1, 1979; Piper, P. J., Tippins, J. R., Morris, H. R., and Taylor, G. W., Advances in Prostaglandin and Thromboxane Research, 6: 121, 1980; Brocklehurst, W. E. Progress in Allergy, 6: 539, 1962; 13. Berry, P. A. and Collier, H. O. J., British Journal of Pharmacology, 23: 201, 1964). Intravenous administration of SRS-A to guinea pigs results in compromised respiration, primarily due to constriction of small peripheral airways (Drazen, J. M. and Austen, K. F., Journal of Clinical Investigation, 53: 1679, 1974). SRS-A also induces vascular permeability when injected intracutaneously in some species, including man (Orange, R. P., Stechschulte, D. J., and Austen, K. F., Federation Proceedings, 28: 1710, 1969). The chemical identity of SRS-A remained unknown until 1979 when it was found to be a mixture of three leukotrienes (LTC4, LTD4, and LTE4) (Murphy, R. C., Hammarstrom, S., and Samuelsson, B. Proceedings of the National Academy of Sciences, U.S.A., 76: 4275, 1979; Morris, H. R., Taylor, G. W., Piper, P. J., and Tippins, J. R., Nature, 285: 104, 1980).
Since this discovery, leukotrienes have been shown to possess all the biological properties described for SRS-A (Lewis, R. A., Drazen, J. M., Austen, K. F., Clark, D. A., and Corey, E. J., Biochemical and Biophysical Research Communications, 96: 271, 1980). Moreover, human lung fragments from patients with extrinsic asthma generate large amounts of leukotrienes when challenged in vitro (Lewis, R. A., Austen, K. F., Drazen, J. M., Clark, D. A., Marfat, A., and Corey, E. J., Proceedings of the National Academy of Sciences, U.S.A., 77: 3710, 1980.) and synthetic leukotrienes are potent constrictors of human airway smooth muscle in vitro (Dahlen, S. E., Hansson, G., Hedqvist, P., Bjorck, T., Granstrom, E., and Dahlen, B., Proceedings of the National Academy of Sciences, U.S.A., 80: 1712, 1983; Dahlen, S., Hedqvist, P., Hammarstrom, S., and Samuelsson, B., Nature, 288: 484, 1980). Aerosolized leukotrienes administered to normal human volunteers cause vigorous airway constriction (Hanna, C. J., Bach, M. K., Pare, P. D., and Schellenberg, R. R., Nature, 290: 343, 1981; Holroyde, M. C., Altounyan, R. E. C., Cole, M., Dixon, M., and Elliott, E. Y., The Lancet, 4: 17, 1981) and LTC4 produces a preferential effect on the peripheral airways which is slow in onset and long in duration (Weiss, J. W., Drazen, J. M., Coles, N., McFadden, E. R., Jr., Weller, P. F., Corey, E. J., Lewis, R. A., and Austen, K. F., Science, 216: 186, 1982). LTC4 levels were found to be elevated in the blood of children undergoing an acute asthmatic attack (Schwartsburg, S. B., Shelov, S. P., and Van Praag, D. Prostaglandins Leukotrienes and Medicine, 26: 143, 1987). Leukotrienes were also detected in sputum of patients with chronic bronchitis (Zakrezewski, J. T., Barnes, N. C., Piper, P. C., Costello, J. F. Prostaglandins, 33: 663, 1987). These pulmonary effects of LTC4 are characteristic of those observed in asthmatic patients following antigen inhalation and are consistent with a major role for leukotrienes in allergic asthma (Lewis, R. A., Chest, 87: 5S, 1985).
2. Allergic Rhinitis. Nasal challenge with specific antigen of patients with allergic rhinitis results in dose- and time-dependent elevations of leukotrienes in nasal washings (Shaw, R. J., Fitzharris, P., Cromwell, O, Wardlaw, A. J., and Kay, A. B., Allergy, 40: 1, 1985). Leukotrienes are proposed mediators of allergic rhinitis as they are stimulators of mucus secretion and vascular permeability (Schelhamer, J. H., Marom, Z., Sun, F., Bach, M. K., and Kaliner, M., Chest, 81 (Suppl): 36, 1982; Coles, S. J., Neill, K. H., Reid, L. M., Austen, K. F., Nii, Y., Corey, E. J., and Lewis, R. A., Prostaglandins, 25: 155, 1983; Soter, N. A., Lewis, R. A., Corey, E. J., and Austen, K. F., The Journal of Investigative Dermatology, 80: 115, 1983), characteristic events in the pathophysiology of this disorder.
3. Rheumatoid Arthritis And Gout. Both LTB4 and 5-HETE stimulate polymorphonuclear leukocyte (PMNL) chemotaxis. LTB4 is one of the most potent chemotactic substances known (Smith, M. J. H., General Pharmacology, 12: 211, 1981). By virtue of their abilities to attract PMNL, these products may contribute to the observed accumulation of PMNL in synovial fluid of individuals with rheumatoid arthritis and gout. 5-HETE and LTB4 have been identified in joint fluids from patients with rheumatoid arthritis (Klickstein, L. B., Shapleigh, C., and Goetzl, E. J., Journal of Clinical Investigation, 66: 1166, 1980; Davidson, E. M., Rae, S. E., and Smith, M. J. H., Journal of Pharmacy and Pharmacology, 34: 410, 1982) and particularly high concentrations of LTB4 have been found in synovial fluids from patients with gout (Rae, S. A., Davidson, E. M., and Smith, M. J. H., The Lancet, 2: 1122, 1982).
4. Psoriasis. LTB4 is present in higher than normal levels in psoriatic lesions (Brian, S. D., Camp, R., Dowd, P., Black, A., and Greaves, M., The Journal of Investigative Dermatology, 83: 70, 1984) which have significantly elevated 5-lipoxygenase activity compared to uninvolved or normal skin (Ziboh, V. A., Casebolt, T. L., Marcelo, C. L., and Voorhees, J. J., The Journal of Investigative Dermatology, 83: 425, 1984). The neutrophil infiltrate that characterizes the early stages of this disease may be due to the chemoattractant properties of LTB4 which can induce micropustule formation when applied topically (VandeKerkhof, P. C. M., Bauer, F. W., and deGroud, R. M., The Journal of Investigative Dermatology, 84: 450, 1985). LTC4 and LTD4 have also been detected in psoriatic skin lesions (Brian, S. D., Camp, R. D. R., Black, A. K., Dowd, P. M., Greaves, M. W., Ford-Hutchinson, A. W., and Charleson, S., Prostaglandins, 29: 611, 1985). These mediators act as vasodilators in human skin and may account for the vasodilation and increased blood flow in psoriatic lesions.
5. Adult Respiratory Distress Syndrome. The presence of elevated LTD4 concentrations in pulmonary edema fluids has led to the suggestion that LTD4 contributes to the permeability defect in the alveolar-capillary barrier in patients with adult respiratory distress syndrome (Matthay, M. A., Eschenbacher, W. L., and Goetzl, E. J., Journal of Clincial Immunology, 4: 479, 1984).
6. Inflammatory Bowel Disease. The colonic mucosa of patients with Crohn's disease has an increased capacity to synthesize sulfidopeptide leukotrienes compared to normal mucosa when exposed to the calcium ionophore A-23187 (Peskar, B. M., Dreyling, K. W., Hoppe, V., Schaarschmidt, K., Goebell, H., and Peskar, B. A., Gastroenterology, 88: 537, 1985). Elevated levels of 5-lipoxygenase products are found in colonic tissue from patients with inflammatory bowel disease; sulfasalazine, a drug used in the treatment of this disease, has been shown to be a weak 5-lipoxygenase inhibitor (Sharon, P. and Stenson, W. F., Gastroenterology, 86: 453, 1984). These observations suggest that increased leukotriene formation may contribute to the characteristic mucosal inflammation of this disorder.
7. Endotoxin Shock. Leukotrienes elicit many of the pathophysiologic symptoms observed in endotoxin shock, such as cardiac depression, increased vascular permeability leading to tissue edema, and increased leukocyte adhesion to endothelial surfaces (Hagmann, W., Denzlinger, C., and Keppler, D. Production of peptide leukotrienes in endotoxin-shock. FEBS Letters, 180: 309, 1985). Furthermore, endotoxins have been shown to trigger the formation of leukotrienes. It has therefore been proposed that leukotrienes play a key role in the lethal action of endotoxin (Konig, W., Scheffer, J. Bremm, K. D., Hacker, J., and Goebel, W., International Archives of Allergy and Applied Immunology, 77: 118, 1985).
8. Ischemia-induced Myocardial Injury. The leukotrienes are potent constrictors of coronary arteries and may play a role in regulating blood flow to the heart. LTC4 and LTD4 exacerbate ischemia-induced myocardial injury in rabbits (Lefer, A. M. Eicosanoids as Mediators of Ischemia and Shock. Federation Proceedings, 44: 275, 1985). Furthermore, infarcted hearts, when reperfused, release larger quantities of leukotrienes in response to stimuli than hearts from sham-operated animals (Barst, S. and Mullane, K., Clinical Research, 33: A516, 1985). These results implicate leukotrienes as potential mediators of ischemia.
9. Central Nervous Pathophysiology. Leukotrienes are synthesized in greater amounts in gerbil forebrains after ischemia and reperfusion (Moskowitz, M. A., Kiwak, K. J., Hekimian, K., et al., Science, 224: 886, 1984), concussive injury, or subarachnoid hemorrhage (subarachnoid injection of blood) (Kiwak, K. J., Moskowitz, M. A., and Levine, L., Journal of Neurosurgery, 62: 865, 1985). The formation of leukotrienes is temporally associated with the cerebral vasospasm and other abnormalities resulting from the insult. Thus a possible role can be suggested for leukotrienes in the pathophysiology resulting from stroke or subarachnoid hemorrhage.
The enzyme 5-lipoxygenase catalyzes the first step leading to the biosynthesis of all the leukotrienes and therefore inhibition of this enzyme provides an approach to limit the effects of all the products of this pathway. Agents capable of abrogating the effects of these potent mediators of pathophysiological processes represent a promising class of therapeutic agents (Brooks, D. W., Bell, R. L., and Carter, G. W. Chapter 8. Pulmonary and Antiallergy Agents, Annual Reports in Medicinal Chemistry, Allen, R. C. ed., Academic Press 1988.